1. Field of the Invention
The present invention relates to satellite communication systems in which Doppler frequency shift may cause serious degradation of demodulation performance and nonnegligible waste of system bandwidth. More particularly, the invention relates to systems providing a feeder link to an earth orbiting satellite transponders such as medium earth orbit satellite (MEOS) communication systems and low earth orbit satellite (LEOS) communication systems having a mobile link illuminating many spot beams with associated access and control and traffic channels for multiple user terminals on the earth where Doppler frequency shift caused by satellite motion is on the order of dozens of kilohertz and also varying with time.
2. Background and Description of Related Art
Typically, the propagation delay and Doppler shift are derived from a radio-frequency (RF) carrier transmitted between mobile communications systems and a moving transceiver, for example, a transceiver included on a moving airplane or satellite. The Doppler shift is a well known physical phenomenon which based on signal propagation delay represents the observed change in frequency of the propagated RF wave that occurs due to the relative motion between the mobile unit and the transceiver. The measured signal propagation delay is the amount of time required for an electromagnetic signal to travel between the communication devices of the system.
Conventional satellite communication systems typically use guard bands at the edges of the useable bandwidth to combat the effects of Doppler shift in earth orbiting satellite systems. This approach causes a considerable waste of the useable system bandwidth in such communication systems, e.g., those employing MEOS and LEOS satellites. For example in a typical system which employs a MEOS satellite, the mobile link Doppler is in the range of approximately +/-11 kHz. To make sure adjacent channels will not overlap, these prior systems use 25 kHz guard bands for every 150 kHz, i.e., having a waste of 25/(25+150)=14.29% of the useable system bandwidth. Where such approaches are applied to LEO satellite communication systems, the percentage of bandwidth waste is expected to be much higher. It would be desireable therefore to provide a system in which effectively 100% of the system bandwidth is used while maintaining zero frequency error.
To this end, it would be desireable first to compensate for the frequency errors dominated by large Doppler frequency shift in satellite communication systems, so that the received frequency at the input of demodulator has negligible error. Secondly, it would be desireable to employ the Doppler frequency shift compensation to effectively use system bandwidth to 100%, i.e., eliminating guard band in conventional systems, which is particularly desirable in wireless communication applications. In MEOS communication systems and LEOS communication systems, Doppler frequency shift caused by satellite motion is on the order of dozens of kilohertz and also varying with time. If not compensated, this Doppler can cause failure of communication or serious demodulation degradation.